The most common elementary illustration of Bayes’ theorem is medical testing for a rare disease. The example is almost a clich´e in probability and statistics books. And yet in my opinion, it’s usually presented too quickly and too abstractly. Here I’m going to risk erring on the side of going too slowly and being too concrete. I’ll work out an example with numbers and no equations before presenting Bayes theorem. Then I’ll include a few graphs. Canonical example of Bayes’ theorem in detail